1. Technical Field
This invention relates to a projection-type display device (projector), and in particular to a projection-type display device having diaphragm-like shielding means to improve contrast.
2. Background Art
There has been widespread adoption of projection-type display devices, in which, in conformance to an electrical signal applied to a spatial light modulation element, light incident on the spatial light modulation element is spatially modulated and emitted, and the emitted light is condensed and projected to display an image. Such a projection-type display device conventionally has a lamp and condensing mirror as a light source, and also has an illumination optical system in which light emitted from the light source is condensed and made incident on the spatial light modulation element; and light from the spatial light modulation element is projected onto a screen or similar by a projection lens.
At present, representative spatial light modulation elements are those having internally a liquid crystal material, which by applying an electric field to the liquid crystals cause rotation of the oscillation direction of the incident polarized light (hereafter called the “liquid crystal type”), and those which have minute moveable mirrors for each pixel, and in which incident light is reflected by the minute moveable mirrors, with spatial modulation performed by changing the retention angles of the minute moveable mirrors according to the image signal (hereafter called the “DMD.RTM. (digital micromirror device) type”).
FIG. 1 shows the basic configuration of a liquid crystal-type projection-type display device (liquid crystal projector). Light is emitted from a light source 21 toward a reflecting mirror 22. Much of the light is condensed at a liquid crystal element (liquid crystal panel) 25 which is the spatial light modulation element by the reflecting mirror 22 and an illumination optical system 23. The condensed light is made incident on a polarizer 24 before being incident on the liquid crystal element 25, to extract light polarized in one direction. Then, an image signal is applied to the liquid crystal element 25, so that the light emitted from the polarizer 24 and incident on the liquid crystal element 25 is spatially modulated, with the polarization direction rotated according to the image signal. Light leaving the liquid crystal element 25 is incident on an analyzer 26, and the light for projection is selected. Light emitted from the analyzer 26 is incident on a projection lens 27, and is projected to display an image on the screen (not shown).
Next, FIG. 2 shows the basic configuration of a DMD.RTM.-type projection-type display device (DMD.RTM. projector). Light is emitted from a light source 31 toward a reflecting mirror 32. Much of the light is condensed at the DMD.RTM. element (DMD.RTM. panel) 34, which is the spatial light modulation element, by the reflecting mirror 32 and an illumination optical system 33. An image signal is applied to the DMD.RTM. element 34, the incident light is spatially modulated, the inclinations of the minute movable mirrors are changed according to the image signal, and the emission direction of light is changed. Light selected by the DMD.RTM. element 34 is incident on a projection lens 35, and is projected onto a screen (not shown) to display an image.
However, in comparisons of the images displayed by projection-type display devices with those of other image display devices, the low contrast of images displayed by projection-type display devices is noted. Here “contrast” means the brightness ratio when a white screen is displayed and when a black screen is displayed.
As shown in FIGS. 1 and 2, even if a projection-type display device is used to display a black screen, a portion of the light, albeit a small amount, is incident on the projection lens. This is because the light source is always being operated.
As a measure to resolve the above inconvenience, in recent years diaphragms have been provided in the illumination optical system or at the projection lens in projection-type display devices (refer to for example Japanese Published Patent Application No. 2001-264728 (paragraphs 0049 to 0054 and FIG. 1)).
The improvement in contrast when an aperture is provided is due to the following reason. In the case of a liquid crystal projector, a characteristic of liquid crystal elements is that the larger the angle of incidence on the liquid crystal panel surface, the greater the degradation in contrast. Consequently in the liquid crystal projector shown in FIG. 1, by providing a diaphragm 41 within the illumination optical system 23 or in proximity to the illumination optical system 23 as shown in FIG. 3, the angle of rays incident on the liquid crystal element 25 can be decreased, so that contrast is improved.
Alternatively, by providing a diaphragm 41 within the projection lens 27 in the liquid crystal projector of FIG. 1, as shown in FIG. 4, so that those rays emitted from the liquid crystal element 25 that are incident on the liquid crystal panel at large angles are blocked by the diaphragm 41, and contrast is improved.
In the case of a DMD.RTM. projector, on the other hand, as explained above, when a black screen is displayed the light incident on the DMD.RTM. element is caused not to be incident on the projection lens. However, because a DMD.RTM. element is an aggregation of minute mirrors, light scattering occurs among the mirrors. Accordingly, there exists some light which ordinarily should not be directed toward the projection lens but is in fact incident on the projection lens. In order to prevent this light from actually being projected insofar as possible, a diaphragm may be provided in the projection lens, so that contrast can be improved.
As explained above, there have been conventional projection-type display devices in which contrast has been improved by installing a diaphragm. However, there is also the inconvenience that use of a diaphragm which blocks a fixed amount of light (for example, an aperture diaphragm with fixed aperture shape) results in a decrease in brightness when displaying a white screen.
As a method for avoiding this inconvenience, a variable diaphragm (diaphragm capable of varying the amount of light to be blocked) may be used, enabling a plurality of states when opening the diaphragm and when blocking light. The problem with contrast in a projected image is due to the brightness of the projection environment. In a bright room, the room brightness (room illumination, sunlight and similar) causes light to be incident on the screen regardless of whether a projection-type display device is present. Hence even if a black screen is displayed, because of outside light, fading of black portions due to the device does not become a problem. White screen brightness exceeds the outside light is necessary.
Conversely, in the case where there is no outside light, fading of a black screen becomes prominent. On the other hand, because the area is dark, brightness in a white screen is not as necessary. This is because human eyes adjust to the brightness.
Hence in an environment in which there is outside light, the diaphragm is opened, white colors are made brighter, and a high-brightness image is presented. On the other hand, in an environment with no outside light, the diaphragm is closed, white is suppressed, and contrast is increased. In this way, by using a variable diaphragm, a balance between brightness and contrast can be achieved.
However, when a variable diaphragm is opened and closed in this way, the angular distribution of light emitted from the spatial light modulation element and arriving at the screen is different when the diaphragm is opened and when it is closed. This is because, as explained above, a portion of the light incident on the spatial light modulation element and a portion of the light emitted from the spatial light modulator is blocked and prevented from reaching the screen. This is the means of increasing the contrast, but as a consequence the following problems may arise.
In a liquid crystal element, the thickness of the portion into which liquid crystals are sealed (the liquid crystal layer) may not be uniform. Even if a voltage at the same level were applied to all the pixels in a liquid crystal panel, because the thickness of the liquid crystal layer differs in different areas, the incident light may not be optically modulated to the same extent. In other words, in the same liquid crystal panel, the graph relating the applied voltage (V) to the transmittance (T) (the VT curve) may not be the same depending on the area of the effective picture plane.
In this state, appropriate light modulation is not possible depending on the location in the picture plane, so that differences arise between the applied image signal and the projected image. For example, even in a case in which an image signal is input at a level corresponding to a transmittance of 50%, the transmittance is not 50% for all locations of the picture plane, so that brightness is uneven in the projected image.
In order to alleviate this brightness unevenness, this applicant has disclosed technology, in Japanese Published Patent Application No. H11-113019, to divide the picture plane of a liquid crystal panel into a plurality of areas, and to perform correction on the image signal applied to the liquid crystal panel according to the VT curve characteristics and similar with respect to each of these areas (hereafter called “uniformity correction technology”).
However, even at the same location in the picture plane, the VT curve changes with the angular distribution of light incident on the liquid crystal panel. Consequently if there are changes, due to opening and closing of the variable diaphragm, in the angular distribution of light reaching the screen after emission from the liquid crystal panel (if opening and closing of a variable diaphragm on the illumination optical system side cause changes in the angular distribution of light incident on the liquid crystal panel, or opening and closing of a variable diaphragm on the projection lens side cause changes in the angular distribution of light projected from the projection lens at the time of incidence on the liquid crystal panel), then even when using such uniformity correction technology, appropriate correction cannot be performed, and brightness unevenness occurs in the projected image.
In the above, cases have been explained in which by opening and closing a variable diaphragm, the angular distribution of light emitted from a liquid crystal panel and reaching a screen changes; however, in addition to the above, reasons for a change in the angular distribution of light emitted from a liquid crystal panel and reaching a screen may include cases in which the zoom position of a projection lens comprising a zoom lens, with variable focal length, is changed, and cases in which the liquid crystal projector has a projection lens which can be replaced, and the projection lens is replaced with a lens having a different f number.
In light of the above, an object of this invention is to provide a projection-type display device which can perform appropriate uniformity correction even in cases when the angular distribution of light emitted from the spatial light modulation element and reaching the screen changes.